In the recent years, there is an increasing demand for digital storage and transmission of audio signals, and, in particular, speech signals. In some cases, like, for example, in mobile communication applications, it is desirable to obtain a comparatively low bitrate.
However, in order to obtain a good compromise between bitrate and audio quality (or speech quality), there are approaches to encode a low frequency portion of an audio signal (for example, a frequency portion up to approximately 6 kHz) using a comparatively high precision, and to rely on a bandwidth extension to reconstruct a high frequency portion of the audio content (for example, above approximately 6 or 7 kHz). For example, the bandwidth extension may be based on a reconstruction of the high frequency portion of the audio content using a comparatively small number of parameters, wherein the parameters may, for example, describe a spectral envelope in a coarse manner.
A well-known implementation of the bandwidth extension is spectral bandwidth replication (SBR), which has been standardized within the MPEG (moving pictures expert group).
For example, some details regarding the spectral bandwidth replication are described in sections 4.6.18 and 4.6.19 of the International Standard ISO/IEC 14496-3:200X(E), subpart 4.
Moreover, reference is also made to US 2011/0099018 A1, which describes an apparatus and a method for calculating bandwidth extension data using a spectral tilt controlled framing. Said patent application describes an apparatus for calculating bandwidth extension data of an audio signal in a bandwidth extension system, in which a first spectral band is encoded with a first number of bits and a second spectral band different from the first spectral band is encoded with a second number of bits, the second number of bits being smaller than the first number of bits. The apparatus has a controllable bandwidth extension parameter calculator for calculating bandwidth extension parameters for the second frequency band in a frame-wise manner for a first sequence of frames of the audio signal. Each frame has a controllable start time instant. The apparatus additionally includes a spectral tilt detector for detecting a spectral tilt in a time portion of the audio signal and for signaling a start time instant for the individual frames of the audio signal depending on a spectral tilt.
However, it has been found that many of the conventional approaches for bandwidth extension substantially degrade an auditory impression which is obtained in the presence of fricatives or affricates. For example, pre-echoes and post-echoes may be caused by conventional bandwidth extension techniques. Moreover, fricatives or affricates may sound too sharp when using conventional bandwidth extension techniques.
In view of this situation, there is a desire to create a concept for a bandwidth extension which allows for an improved audio quality.